Nucleic acid sequence encoding apamin binding protein

ABSTRACT

The present invention relates to an isolated nucleic acid fragment comprising a nucleic acid sequence encoding an apamin receptor protein, or biologically active fragment thereof.

RELATED APPLICATION

This application is a continuation of co-pending application Ser. No. 07/923,095 filed Jul. 30, 1992, now U.S. Pat. No. 5,401,652, which is incorporated herein by reference in its entirety.

Potassium (K) channels are integral membrane proteins of great molecular and functional diversity, present in practically all mammalian cells. These channels are primarily responsible for maintaining a resting membrane potential and are rapidly activated in response to an external depolarizing stimulus, binding of certain ligands, or changes in the intracellular concentration of calcium or ATP. In the excitable cells such as neurons or cardiac myocytes, K-channels determine the duration of the action potential thus performing a vital function in the CNS and the cardiac functions (reviewed in 1-2) The calcium-activated K-channel sub-family consists of at least three discernible ionic currents; a large ("BK"), an intermediate ("IK") and a small conductance ("SK") channels (Reviewed in 3-5). These K-channels are activated in response to a rise in the intracellular concentration of calcium [Ca²⁺ ]i. In addition to [Ca²⁺ ]i, the "BK" and "IK" channels are also sensitive to the changes in the membrane potential, whereas "SK" channel has no significant voltage sensitivity.

Functionally, the SK-channel is involved in the afterhyperpolarization that follows action potentials in many neurons. These include the sympathetic ganglionic neurons, hippocampal neurons, neurosecretory neurons and spinal motoneurons, as well as the skeletal muscle cells (1, 5-9). Furthermore, the SK-channel has been suggested to play a major role in the spontaneous transient outward currents in the tracheal smooth muscle cells (10), the inhibitory action of the α₁ -adrenoceptors, neurotensin receptor and the P₂ -subtype of the ATP receptor (4, 9).

The neuronal and the skeletal muscle SK-channel is specifically and avidly blocked by a bee venom-derived peptide toxin, apamin (5, 11-14). By all indications, the apamin receptor complex is either identical to, or closely associated with the SK-channel. Apamin is an 18 amino acid neurotoxic peptide which has a single class of binding sites in the rat brain synaptosomes and brain slices with an apparent dissociation constant (K_(d)) of 10-25 pM (15, 16). Apamin is also capable of a temperature dependent and high affinity (K_(d) =30-150 pM) binding to the detergent solubilized brain receptor sites (17-20). The reported B_(max) value for the rat brain synaptosomes and brain slices is 10-30 fmol/mg protein (16, 17, 20), while that for the detergent solubilized receptor ranges from 0.45 to 17 fmol/mg protein (18, 19).

The polypeptide components of the apamin receptor have been studied by several groups. Cross-linking experiments using [¹²⁵ I] apamin, followed by SDS-PAGE and autoradiography have indicated that the apamin binding proteins of the rat brain synaptosomal membrane consist of two protein species, a major 80-86 KDa protein and, in most reported preparations, a minor 50-59 KDa band (17, 21, 22). Partial peptide mapping of the two protein bands, using an anti-apamin anti-serum, has shown that the smaller polypeptide is likely to be a proteolytic fragment of the larger protein and not an additional subunit of the apamin binding protein in the brain. Furthermore, in the plasma membrane of the cultured neurons or astrocytes, there are additional components with the ability to cross-link to [¹²⁵ I] apamin. Cross-linking of [¹²⁵ I] apamin to the membranes from the rat heart, liver and smooth muscle has also indicated that a 85-87 KDa polypeptide is the major labeled component of the apamin binding complex (23). A second 59 KDa protein was identified in the liver membrane only (23).

The blocking of the small conductance calcium activated potassium channel (sKca) results in prolongation of the action potential, while its activation by an increase in the intracellular calcium concentration accelerates the rate of hyperpolarization, thus shortening the duration of the action potential. In vascular smooth muscle cells (such as those in veins and arteries), activation of sKca results in the hyperpolarization of the smooth muscle membrane, which in turn results in the inhibition of the voltage-gated calcium channels. The inhibition of the latter may then lead to the relaxation of the blood vessels and lowering of the blood pressure. In the heart, modulation of sKca can be a potentially useful means to regulate an arrhythmic heart. In the nervous system, the hippocampus of Alzheimer's patients shows a drastic reduction in apamin denisty (30). Further apamin receptor in neurons has been reported to be involved in the process of learning and memory (42). Thus, manipulation of this receptor may also result in improving cognition. Notwithstanding the significant therapeutic potential manipulation of sKca may have, relatively little is known about the identity of the proteins involved in this channel. The present invention now provides a key element in the study of the potassium channel function.

SUMMARY OF THE INVENTION

The present invention relates to a nucleic acid fragment comprising a sequence encoding an apamin receptor, as well as the recombinantly produced apamin receptor per se. Such receptors are associated with calcium activated potassium channels in a variety of animal tissues, such as brain, skeletal, cardiac, vascular smooth muscle, pancreas, kidney and liver tissue. An exemplary sequence of Kcal 1.8, a porcine receptor, is provided in FIGS. 3A-3E; however, the invention also encompasses any nucleotide sequence which hybridizes, under medium or high stringency conditions (as defined in the Examples below), with a nucleotide sequence encoding the amino acid sequence of FIGS. 3A-3E, as well as the biologically active proteins and fragments encoded by such sequences. By "biologically active" is meant proteins or fragments which are capable of eliciting production of antibodies capable of binding to the receptor, as well as proteins or fragments which are associated with calcium activated K+ channels (such as "BK" or "IK") but do not necessarily bind apamin.

The invention also relates to host cells and recombinant vectors useful in expressing the apamin receptor gene and protein. Such hosts will provide a convenient basis for development of screens designed to identify compounds which are capable of modulating activity of the receptor and thus, modulate the activity of the potassium channel. In the heart, modulation of this channel provides a means for regulating an arrhythmic heart; thus, any drug that can open or close this potassium channel is considered a potential antiarrhythmic agent. Similarly, in vascular smooth muscle cells, such as those in veins and arteries, activation of the potassium channel results in hyperpolarization of the smooth muscle membrane, which in turn results in the inhibition of the voltage-gated calcium channels. The inhibition of the latter will then lead to relaxation of the blood vessels and lowering of blood pressure. The receptor is also associated with cognition functions. As noted above, receptor density decreases in Alzheimer's patients, and is involved in the process of learning and memory. Thus, compounds which activate the receptor may be useful in improving impaired cognitive function in Alzheimer's patients, or to enhance memory and learning capacity. Therefore, a convenient system enabling the detection of compounds that modulate potassium channel activity has the potential for identifying drugs with tremendous therapeutic utility. Also, the isolated nucleic acid sequence detectably labelled can be used as a diagnostic probe for Alzheimer's disease, by determining the level of expression of such receptors in peripheral neurons of individuals suspected of being affected. Copending and cofiled applications which have as common inventors Mohammad Rena Ziai and Patricia Tyson Sokol, relating to purified apamin binding proteins [Ser. No. 07/922,307 now abandoned], and affinity matrix for binding protein purification [Ser. No. 07/922,604 now abandoned], are each incorporated herein by reference in their entirety.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1: Northern blotting of the mRNA encoding the apamin receptor.

(1A) Poly A⁺ -mRNA isolated from adult rat brain (lane 1), or bovine brain (lane 2) or porcine brain (lane 3) are separated on a denaturing agarose gel, blotted onto nitrocellulose, hybridized with ³² P-labelled Kcal 1.6 cDNA and autoradiographed.

(1B) Poly A⁺ -mRNA isolated from the neonatal rat brain are separated on a denaturing agarose gel, blotted onto nitrocellulose, hybridized with ³² P-labelled Kcal 1.6 cDNA and autoradiographed. The arrows indicate the size (in kilobases) of the two hybridized mRNA bands.

FIG. 2: Genomic Southern hybridization analysis of Kcal 1.6. EcoRI cut-genomic DNA from human (lane 1), monkey (lane 2), rat (lane 3), mouse (lane 4), dog (lane 5), cow (lane 6), rabbit (lane 7), chicken (lane 8) and yeast (lane 9) are hybridized with ³² P-labelled Kcal 1.6 cDNA and autoradiographed.

FIGS. 3A-3E:

The nucleotide sequence (SEQ ID NO. 1) and its amino acid translation (SEQ ID NO. 2) of Kcal-1.8 cDNA. The underlined amino acids indicate the potential transmembrane domains of the protein. The oval represents the potential site for protein kinase C. The (*) indicate amino acids which form a potential calcium binding site.

FIG. 4 The hydropathy plot for the protein encoded by Kcal-1.8 cDNA. The four putative but strong hydrophobic domains are indicated by arrows.

FIG. 5: Binding of apamin to plasma membrane of CV-1 cells transfected with Kcal-1.8 cDNA in a pRC/CMV vector.

DETAILED DESCRIPTION OF THE VECTOR

A full-length apamin binding protein nucleic acid sequence, presumed to be associated with a calcium activated K⁺ channel, is first isolated from a porcine vascular smooth muscle (aorta) expression cDNA library in a λ-ZAP vector. The library is screened with polyclonal sera raised against a bovine brain apamin receptor. Screening of about 2 million plaque forming units yields four positive plaques which are rescreened and plaque purified.

The λ-ZAP is transformed into "pBluescript" plasmid by standard techniques, the DNA is digested with the restriction endonucleases EcoRI and XhoI to release the cDNA inserts and analyzed by agarose gel electrophoresis. One 1.6 Kb cDNA clone (designated Kcal 1.6) is selected for Northern hybridization, genomic Southern blotting and DNA sequencing. As shown in FIG. 1A, the cDNA Kcal 1.6 detects a single mRNA band of approximately 2.1 Kb in the adult rat brain mRNA (lane 1), bovine brain mRNA (lane 2) and porcine brain mRNA (lane 3). The probe, however, reveals two mRNA bands of 2.1 and 3.0 Kb in size in the Northern blot of mRNA from neonatal rat brain (FIG. 1B). These results suggest that in the neonatal rat brain, there are two distinct mRNA species which hybridize to Kcal 1.6, possibly arising by the alternate splicing of mRNA.

Next, an EcoRI cut-genomic Southern blot is probed with Kcal 1.6 cDNA. As shown in FIG. 2, after repeated washing of the blot at high stringency, the Kcal 1.6 probe detects a single 14 Kb band in human (lane 1) and in monkey (lane 2). However, there are variable patterns of hybridization in the rat (lane 3), mouse (lane 4), canine (lane 5), bovine (lane 6), rabbit (lane 7) and chicken (lane 8). There is no detectable hybridization with the yeast DNA (lane 9). This experiment indicates that there are significant sequence homologies among the genes encoding p80 in various species. Furthermore, the gene(s) encoding p80 in human and monkey are probably more similar than those in other species.

Kcal 1.6 cDNA is then sequenced. The nucleotide sequence obtained indicates that the clone is not of full length and lacks the initiation methionine residue. To obtain a full-length clone, Kcal 1.6 is used as a probe and the original porcine aorta cDNA library is screened, and positive clones analyzed by restriction mapping and electrophoresis for relatedness and insert size. One cDNA clone (designated Kcal 1.8), which appears to be slightly longer than Kcal 1.6, is selected and sequenced by a Taq polymerase sequencing technique. When the nucleotide sequence is translated in frame, the cDNA Kcal 1.8 encodes a protein of 437 amino acids (FIGS. 3A-3E), with an initiation methionine and a stop site. Hydrophobicity analysis (FIG. 4) of the sequence indicates the presence of four strongly hydrophobic putative transmembrane domains (TMD1-4), a short amino terminus and a long carboxyl terminus. The sequence has some interesting features. It contains a strong "EF-Hand" consensus sequence (in FIG. 3B, indicated by a *). The EF-Hand consensus sequence is present in virtually all calcium binding protein members of calmodulin and troponin C families. In fact, the EF-Hand motif in Kcal 1.8 almost perfectly matches that of calmodulin, as well as a recently cloned component of Drosophila calcium activated K-channel, "Slo" (28). In addition, the sequence flanking the putative "EF-Hand" motif of Kcal 1.8 has significant homology with a number of known calcium binding proteins including troponin C, myosin, calreticulin, PEP-19, and several others. Since the small conductance calcium-activated potassium channel (skca) must have a calcium binding site, it gives further support to the belief that Kcal 1.8 indeed encodes skca. If the "EF-Hand" motif is in fact a calcium binding site of Kcal 1.8 protein, it places the "EF-Hand" motif on the cytoplasmic side of the membrane. The amino acid sequence of Kcal 1.8 also contains one protein kinase C site, and one tyrosine kinase phosphorylation site (not shown). In addition, a "leucine zipper" motif can be identified in the C-terminal portion of the protein (FIG. 3E, boxed "L"). At present, the significance, if any, of this motif in Kcal 1.8 is unclear. However, the presence of these putative phosphorylation sites, together with the "EF-Hand" motif are likely to place both N- and C-termini of the protein in the cytoplasmic side of the plasma membrane.

To further confirm Kcal 1.8's identity as an apamin receptor, Kcal cDNA is introduced into a stable mammalian expression vector, pRC/CMV, which is used to transfect CV-1 cells (African green monkey kidney). Cells stably expressing the Kcal 1.8 gene product are selected and are contacted with radiolabelled apamin, in the presence or absence of unlabelled apamin. A number of transfectants show enhanced binding of radio-labelled apamin, thereby adding further confirmation of Kcal 1.8's identity.

The foregoing discussion, and the sequences provided in FIGS. 3A-E relate to a porcine smooth muscle apamin receptor. However, it will be understood that the invention encompasses more than the specific exemplary sequences. Modifications to the sequence, such as deletions, insertions, or substitutions in the sequence which produce silent changes in the resulting protein molecule are also contemplated. For example, alteration in the gene sequence which reflect the degeneracy of the genetic code, or which result in the production of a chemically equivalent amino acid at a given site, are contemplated; thus, a codon for the amino acid alanine, a hydrophobic amino acid, may be substituted by a codon encoding another less hydrophobic residue, such as glycine, or a more hydrophobic residue, such as valine, leucine, or isoleucine. Similarly, changes which result in substitution of one negatively charged residue for another, such as aspartic acid for glutamic acid, or one positively charged residue for another, such as lysine for arginine, can also be expected to produce a biologically equivalent product. Nucleotide changes which result in alteration of the N-terminal and C-terminal portions of the protein molecule would-also not be expected to alter the activity of the protein. It may also be desirable to eliminate one or more of the cysteines present in the sequence, as the presence of cysteines may result in the undesirable formation of multimers when the protein is produced recombinantly, thereby complicating the purification and crystallization processes. In some cases, it may in fact be desirable to make mutants of the sequence in order to study the effect of alteration on the biological activity of the protein. Each of the proposed modifications is well within the routine skill in the art, as is determination of retention of biological activity of the encoded products.

The invention also encompasses homologous sequences obtained from other species and other tissues. As has already been demonstrated above, the nucleic acid sequence depicted in FIGS. 3A-E hybridizes, under relatively stringent conditions, with nucleic acid fragments present in a number of other species, including human, thus demonstrating the ability to isolate other non-porcine sequences. Moreover, apamin receptors from tissue types other than vascular smooth muscle are also known to exist. Brain, skeletal muscle, and liver, in addition to vascular smooth muscle, have been repeatedly demonstrated to express a single class of binding site (4, 15-20). On the other hand, cardiac tissue seems to exhibit a heterogeneous population of target sites. The sequence disclosed in FIGS. 3A-E can thus be used as a probe to isolate the corresponding receptors from other species and tissues. Alternate receptor types are isolatable as follows. cDNA libraries prepared from mRNA from the specific tissue type of interest are probed with radiolabelled Kcal 1.8 cDNA and washed under medium stringency (e.g., 1×SSC, 0.1% SDS, 55° C.). Plaques which appear positive are rescreened to verify authenticity. The positive plaques are then used in plasmid rescue according to techniques known in the art. Rescued plasmids are purified, cut with appropriate restriction enzymes, and analyzed in an agarose gel stained with ethidium bromide. The second gel is transferred to an nitrocellulose filter, probed with labelled Kcal 1.8, washed sequentially under a medium, then high stringency (0.1×SSC, 0.1% SDS, at 65° C.) wash and exposed to X-ray film. Those inserts which strongly hybridize to Kcal 1.8 under high stringency conditions represent likely receptor cDNA candidates. Further confirmation of the identity of these putative receptors can be accomplished according to the protocols described in the following examples, or in accordance with routine techniques known in the art. Thus, the invention encompasses not only the nucleotide and amino acid sequences depicted in FIGS. 3A-E, but also nucleotide sequences which hybridize, under medium or high stringency conditions, with nucleotide sequence encoding the amino acid sequence of FIGS. 3A-E, as well as the biologically active proteins or fragments encoded thereby.

The nucleic acid sequence can be used to express the receptor protein in a variety of host cells, both prokaryotic and eukaryotic for the chosen cell line. Examples of suitable eukaryotic cells include mammalian cells, plant cells, yeast cells, and insect cells. Suitable prokaryotic hosts include Escherichia coli and Bacillus subtilis.

Suitable expression vectors are selected based upon the choice of host cell. Numerous vectors suitable for use in transforming bacterial cells are well known. For example, plasmids and bacteriophages, such as λ phage, are the most commonly used vectors for bacterial hosts, and for E. coli in particular. In both mammalian and insect cells, virus vectors are frequently used to obtain expression of exogenous DNA. In particular, mammalian cells are commonly transformed with SV40, polyoma virus, or transfected with plasmids such as pRC/CMV; and insect cells in culture may be transformed with baculovirus expression vectors. Yeast vector systems include yeast centromere plasmids, yeast episomal plasmids and yeast integrating plasmids. The invention encompasses any and all host cells transformed or transfected by the claimed nucleic acid fragments, as well as expression vectors used to achieve this. In particular, the host cells chosen for transfection are cells which exhibit only low (i.e., background) levels of receptor expression (e.g., see FIG. 5) before transcription.

In a preferred embodiment, nucleic acid sequences encoding an apamin receptor are used to transfect eukaryotic cells, preferably mammalian cells. For an initial determination of the ability of a given sequence to produce an apamin binding protein, transient expression, using plasmids such as pcDNAI or PSG5 into which the putative receptor DNA sequence has been ligated, and CMT-1 or COS-1 or -7 cells, can be employed. CMT-1 cells are transfected using the calcium phosphate precipitation method, and within 24 hours of transfection, the SV40 large T antigen is induced with addition of zinc to the medium. Seventy-two hours after transfection, cells are harvested for either RNA isolation or apamin binding assays. Expression is compared between cDNA and mock-transfected cells to determine if receptor activity is achieved by transfected cells. A positive host cell is preferably one which exhibits about twice the background level of apamin binding observed in non-transfected host cells of the same type.

For use of the sequences in screen development, stable expression of the DNA may be desirable. In this case, the DNA encoding the receptor is ligated into a stable vector containing a selectable marker, such as pRC/CMV, pcDNAI Neo, pXTI, or pMAM Neo. The plasmid DNA is linearized and introduced into an appropriate cell line for such vectors, e.g., CV-1, CHO, HepG-2 or NIH3T3 cells, by electroporation. Successfully transfected cells are identified by selection and isolated clones are picked and amplified. To determine transcription of Kcal message, cellular RNA is isolated and separated electrophoretically on agarose gel. Detection of endogenous and exogenous mRNA is accomplished using Kcal 1.8 as a probe.

Identification of exogenous (transfected) mRNA is accomplished by probing with a 400 bp fragment from the 5' untranslated region of cDNA, since this region is most divergent among species, diminishing the incidence of cross-hybridization.

The ability of any given isolated DNA sequence to yield a functional apamin receptor is determinable by a simple apamin binding assay. Transfected cells are prepared as previously described (41). Binding assays are performed by a standard procedure (16), and values for maximum binding of ligand to receptor (Bmax) and dissociation constant (Kd) for each cell line is calculated.

Further evaluation of the measurement of potassium channel activity in cultured transfectant cells is accomplished by ⁸⁶ Rb efflux assay (30, incorporated herein by reference). Briefly, stably transfected cells are loaded overnight with ⁸⁶ Rb in microtiter plates; the medium is then discarded and adherent cells washed three times to remove isotope. Cells are then incubated for 30 minutes at 37° C. with an isotonic buffer containing 20 mM CaCl₂ and 100 μM calcium ionophore A23187. The supernatants from wells are recovered and counted. The cell layer is solubilized in Triton X-100 and also counted, and the percent efflux of ⁸⁶ Rb calculated as described. The experiment is carried out in the presence or absence of 1 mM apamin (an sKca blocker) or 1 μM charybdotoxin (a BKca blocker), and control experiments carried out in parallel with cells being incubated with buffer, but without added ionophore. The percent efflux in transfectants harboring cloned DNA mock transfectants, and wild-type CV-1 cells (to measure endogenous efflux) are compared. Such assays are also useful in determining the effect of structural change in the channel in its function, and also to evaluate functional differences between different receptor subtypes. This assay is useful both in confirming activity of a putative receptor/channel as well as confirming the effects

DEPOSIT OF BIOLOGICAL MATERIALS

The following biological materials were deposited with the American Type Culture Collection, 12301 Parklawn Drive, Rockville, Md., on Jun. 18, 1992, and given the Accession Numbers indicated:

    ______________________________________                                         Material             Accession No.                                             ______________________________________                                         E. coli containing   ATCC 69017                                                pBluescript plasmid                                                            containing Kcal 1.8                                                            ______________________________________                                    

The present invention is further illustrated by the following non-limiting examples.

EXAMPLES

1. Screening Expression Library

A porcine aorta expression cDNA library in λ-Uni ZAP λR (Stratagene, La Jolla, Calif.) is probed with a 1:1000 dilution of a murine anti-apamin binding protein polyclonal antiserum (M2) using the Vectastain ABC kit (Vector Laboratories Inc., Burlingame, Calif.) as the secondary antibody and detection system. Approximately 2×10⁶ plaque forming units are screened in this manner.

Four positive plaques are selected from the first round of screening. These are subjected to a re-screen and plasmids (pBluescript) containing the cDNA inserts are rescued using a helper phage. The parent plasmid DNA is digested with the restriction endonucleases EcoRI and XhoI to release the cDNA inserts and analyzed by agarose gel electrophoresis. One 1.6 Kb cDNA clone (designated Kcal 1.6) is selected for Northern hybridization, genomic Southern blotting and DNA sequencing. For Northern hybridization, polyA mRNA is isolated from frozen rat tissues using "Fast Track" mRNA isolation kit (Invitrogen, San Diego, Calif.) or purchased from Clontech Labs (Palo Alto, Calif.). Genomic Southern blot, "Zoo-blot" is purchased from Clontech Labs and processed as described by the manufacturer. As shown in FIG. 1A, the cDNA Kcal 1.6 detects a single mRNA band of approximately 2.1 Kb in the adult rat brain mRNA (lane 1) bovine brain mRNA (lane 2) and porcine brain mRNA (lane 3). The probe, however, reveals two mRNA bands of 2.1 and 3.0 Kb in size in the northern blot of mRNA from neonatal rat brain (FIG. 1B). These results may indicate that in the neonatal rat brain, there are two distinct mRNA species which hybridize to Kcal 1.6, possibly arising from the alternate splicing of mRNA. Next, an EcoRI cut-genomic southern blot is probed with Kcal 1.6 cDNA. As shown in FIG. 2, after repeated washing of the blot at high stringency, the Kcal 1.6 probe detects a single 14 Kb band in human (lane 1) and in monkey (lane 2). However, there are variable patterns of hybridization in the rat (lane 3), mouse (lane 4), canine (lane 5), bovine (lane 6), rabbit (lane 7) and chicken (lane 8) ranging from 14 Kb to 3.0 Kb. There is no detectable hybridization with the yeast DNA (lane 9). These results indicate that there are notable homologies among the genes encoding p80 in various species.

2. Sequencing of Kcal 1.6

DNA sequencing is performed using the "Taq-Track" sequencing system (Promega Corp.) or the "Sequenase" system (U.S. Biochemical, Cleveland, Ohio). The nucleotide sequence obtained indicates that the clone is not full length, and lacks an initiation methionine residue. To obtain a full-length clone, Kcal 1.6 is used as a probe to screen the original porcine aorta cDNA library. Positive clones are analyzed by restriction mapping and electrophoresis for relatedness and insert size. One cDNA clone (designated Kcal 1.8) which is slightly longer than Kcal 1.6 is isolated and sequenced. The nucleotide and amino acid sequence of Kcal 1.8 is shown in FIGS. 3A-E. The cDNA encodes 437 amino acids, the hydropathy plot (FIG. 4) indicates four strongly hydrophobic putative transmembrane domains. There is a putative calcium binding domain which closely matches that of the cloned cDNA slo encoding a putative calcium activated K-channel in Drosophila. However, there is no significant sequence homology between Kcal 1.8 and slo in other regions.

There is one strong consensus sequence in Kcal 1.8 for the cAMP dependent protein kinase, as well as those putative casein kinase phosphorylation sites. The Kcal 1.8 sequence has no significant homologies with any known voltage gated K-channels, sodium channels or calcium channels.

3. Expression of Kcal 1.8

CV-1 cells (ATCC CCL70) stably expressing the Kcal 1.8 gene product are produced by introducing the cDNA in the stable mammalian expression plasmid, pRc/CMV (InVitrogen) which contains a Neo^(r) marker. The Kcal 1.8 sequence is extracted from the pBluescript vector by digestion, with EcoRI and XhoI, and ligated into the corresponding sites of pRc/CMV. To transfect the cells, confluent 100 nun dishes of CV-1 cells are split and replated the day before the transfection, to ensure the cells are in log-growth phase. For electroporation, cells are harvested with trypsin, washed once with phosphate-buffered saline, and twice with an isotonic, low ionic strength buffer containing 272 mM sucrose, 7 mM sodium phosphate, pH 7.4 and 1 mM MgCl₂ (buffer E). The cells are resuspended in this same buffer to a final concentration of 1.5×10⁶ cells/ml. Twenty μg of the appropriate vector are digested with 40 units of ScaI for 2 hours at 37° C. to linearize the plasmid. The linearized plasmid is phenol/chloroform extracted, EtOH precipitated, and resuspended in 400 μl of Buffer E. The resuspended DNA is mixed with 400 μl of CV-1 cells (1×10⁶ cells) and incubated at room temperature for 2 minutes prior to electroporation. Electroporation is accomplished using a Bio-Rad gene pulser with a 300-V pulse at 25 μFarads. Transfections are done in duplicate. The suspension is allowed to further incubate for 5 minutes at room temperature, and then plated onto 100 mm tissue culture dishes with 10 mls of Dulbecco's modified Eagle's medium containing 10% fetal calf serum. Two days following transfection, G418 is added to a final concentration of 200 ug/ml. When isolated G418-resistant colonies are identified, they are selected with cloning cylinders and amplified.

Transfected cells are harvested and washed. They are incubated with [¹²⁵ I]apamin in the binding buffer "B": Tris-HCl 10 mM, KCl 1 10 mM, pH 7.4, in the presence or absence of 1 μM cold apamin. The incubation is at 4° C. for 30 minutes with cold apamin, followed by 1 hour incubation at 4° C. with [¹²⁵ I]apamin (20,000 cpm/well). Target cells are then filtered and washed with the binding buffer plus BSA. The filters are counted in a gamma counter.

As shown in FIG. 5, Transfectant #1, 3, 10 and 12 show significantly enhanced binding of [¹²⁵ I]apamin, compared to other transfectants shown.

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    __________________________________________________________________________     SEQUENCE LISTING                                                               (1) GENERAL INFORMATION:                                                       (iii) NUMBER OF SEQUENCES: 2                                                   (2) INFORMATION FOR SEQ ID NO:1:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 1730 base pairs                                                    (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: cDNA                                                       (iii) HYPOTHETICAL: NO                                                         (iv) ANTI-SENSE: NO                                                            (vi) ORIGINAL SOURCE:                                                          (A) ORGANISM: Pig                                                              (ix) FEATURE:                                                                  (A) NAME/KEY: CDS                                                              (B) LOCATION: 140..1456                                                        (xi) SEQUENCE DESCRIPTION: SEQ ID NO:1:                                        AGCAGCTCCATAGGCCCAGCCCCGGCGTACAAGGATCACTTCCGGTGGTACTTCACTACC60                 AAGAAGCTGCGATTGGGCGAGCGTGGAAGGGGCATTTCCGGTGTCCACCTGCTTGGGTTC120                TTTGGACAGAAGTAGGAAGATGGAGCTCGGCGCCGCGGCCCGTGCTTGGTCG172                        MetGluLeuGlyAlaAlaAlaArgAlaTrpSer                                              1510                                                                           CTCTTGTGGCTGCTGCTTCCCTTGCTTGGCCTGGTCGGCGCCAGCGGT220                            LeuLeuTrpLeuLeuLeuProLeuLeuGlyLeuValGlyAlaSerGly                               152025                                                                         CCCCGTACCTTAGTGCTTCTGGACAACCTCAACCTGCGGGAGACGCAT268                            ProArgThrLeuValLeuLeuAspAsnLeuAsnLeuArgGluThrHis                               303540                                                                         TCACTTTTCTTCCGGAGCCTAAAGGATCGGGGCTTCGTACTCACATTC316                            SerLeuPhePheArgSerLeuLysAspArgGlyPheValLeuThrPhe                               455055                                                                         AAGACCGCAGATGACCCCAGCCTGTCCCTGATTAAGTACGGAGAGTTC364                            LysThrAlaAspAspProSerLeuSerLeuIleLysTyrGlyGluPhe                               60657075                                                                       CTCTATGACAATCTCATCGTCTTTTCACCTTCGGTAGAAGATTTTGGA412                            LeuTyrAspAsnLeuIleValPheSerProSerValGluAspPheGly                               808590                                                                         GGAAATATCAACGTGGAGACCATCAGTACCTTTATCGACGGCGGAGGC460                            GlyAsnIleAsnValGluThrIleSerThrPheIleAspGlyGlyGly                               95100105                                                                       AGTGTCCTGGTAGCTGCCAGCTCAGACATCGGTGACCCTCTCCGCGAG508                            SerValLeuValAlaAlaSerSerAspIleGlyAspProLeuArgGlu                               110115120                                                                      CTGGGCAGTGAGTGTGGGATTGAGTTTGACGAGGAGAAAACGGCCGTC556                            LeuGlySerGluCysGlyIleGluPheAspGluGluLysThrAlaVal                               125130135                                                                      ATTGACCATCACAACTATGATGTCTCAGACCTCGGCCAGCACACGCTC604                            IleAspHisHisAsnTyrAspValSerAspLeuGlyGlnHisThrLeu                               140145150155                                                                   ATTGTGGCCGACACTGAGAACCTGCTGAAGGCCCCGACCATCGTCGGG652                            IleValAlaAspThrGluAsnLeuLeuLysAlaProThrIleValGly                               160165170                                                                      AAGTCATCTCTGAATCCCATCCTCTTCCGAGGTGTTGGGATGGTGGCT700                            LysSerSerLeuAsnProIleLeuPheArgGlyValGlyMetValAla                               175180185                                                                      GATCCTGACAATCCTTTGGTGCTGGACATCCTGACCGGCTCTTCTACC748                            AspProAspAsnProLeuValLeuAspIleLeuThrGlySerSerThr                               190195200                                                                      TCTTACTCCTTCTTCCCAGATAAACCCATCACGCAGTACCCGCACGCG796                            SerTyrSerPhePheProAspLysProIleThrGlnTyrProHisAla                               205210215                                                                      GTGGGGAAGAACACGCTGCTCATCGCGGGGCTGCAGGCCCGGAACAAC844                            ValGlyLysAsnThrLeuLeuIleAlaGlyLeuGlnAlaArgAsnAsn                               220225230235                                                                   GCCCGTGTCATCTTCAGCGGCTCCCTCGACTTCTTCAGCGATGCCTTC892                            AlaArgValIlePheSerGlySerLeuAspPhePheSerAspAlaPhe                               240245250                                                                      TTCAACTCCGCGGTGCAGAAGGCCACGCCTGGCTCCCAGAGGTATCCC940                            PheAsnSerAlaValGlnLysAlaThrProGlySerGlnArgTyrPro                               255260265                                                                      CAGACAGGCAACTATGAGCTCGCCGTGGCCCTCTCCCGCTGGGTGTTC988                            GlnThrGlyAsnTyrGluLeuAlaValAlaLeuSerArgTrpValPhe                               270275280                                                                      AAGGAGGAGGGTGTCCTCCGAGTGGGGCCTGTGTCCCACCATCGGGTG1036                           LysGluGluGlyValLeuArgValGlyProValSerHisHisArgVal                               285290295                                                                      GGCGAGAAAGCCCCACCCAACGCCTACACCGTCACTGACCTAGTCGAG1084                           GlyGluLysAlaProProAsnAlaTyrThrValThrAspLeuValGlu                               300305310315                                                                   TACAGCATCGTGATTGAGCAGCTCTCACAGGGCAGATGGGTCCCCTTT1132                           TyrSerIleValIleGluGlnLeuSerGlnGlyArgTrpValProPhe                               320325330                                                                      GATGGCGACGACATTCAGCTGGAGTTTGTCCGCATCGATCCTTTCGTG1180                           AspGlyAspAspIleGlnLeuGluPheValArgIleAspProPheVal                               335340345                                                                      AGGACCTTCTTGAAGAGGAAAGGCGGCAAGTACAGCGTCCAGTTCAAG1228                           ArgThrPheLeuLysArgLysGlyGlyLysTyrSerValGlnPheLys                               350355360                                                                      TTGCCGGACGTGTACGGCGTGTTCCAGTTCAAAGTGGACTACAACCGG1276                           LeuProAspValTyrGlyValPheGlnPheLysValAspTyrAsnArg                               365370375                                                                      CTGGGCTACACGCACCTGTACTCCTCCACTCAGGTGTCCGTGCGGCCC1324                           LeuGlyTyrThrHisLeuTyrSerSerThrGlnValSerValArgPro                               380385390395                                                                   CTGCAGGCACACGCAGTACGAGCGCTTCATCCCCTCGGCTTACCCCTA1372                           LeuGlnAlaHisAlaValArgAlaLeuHisProLeuGlyLeuProLeu                               400405410                                                                      CTACGCCAGCGCCTTCTCCATGATGGTCGGGCTCTTCATCTTCAGCGT1420                           LeuArgGlnArgLeuLeuHisAspGlyArgAlaLeuHisLeuGlnArg                               415420425                                                                      CGTCTTCTTGCACATGAAGGAGAAGGAGAAGTCTGACTGAGGGGCCGGGCCGG1473                      ArgLeuLeuAlaHisGluGlyGluGlyGluVal                                              430435                                                                         GCCCCAGGACTCCTTACAACACACAGGGAGGGTTTTTATAGGCTTGCCTTCCCCCCCCTT1533               TATGGTGGGCTTTGTTTGTTTTTAAAGCCACGGACAATGGCACAGCTTACCTCAGTGGGA1593               GATGCAAGATGAGTACCAGGGGGTGGTTAGGAATAATTTCTAAGTTTTTCCACCTTGAAT1653               GCTGAGTGGTATTTTTCATATGTAAAGTCAACTGATTTCTAAAATAAAAGAAAAACATCA1713               CCCTCAGAAAAAAAAAA1730                                                          (2) INFORMATION FOR SEQ ID NO:2:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 438 amino acids                                                    (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: protein                                                    (xi) SEQUENCE DESCRIPTION: SEQ ID NO:2:                                        MetGluLeuGlyAlaAlaAlaArgAlaTrpSerLeuLeuTrpLeuLeu                               151015                                                                         LeuProLeuLeuGlyLeuValGlyAlaSerGlyProArgThrLeuVal                               202530                                                                         LeuLeuAspAsnLeuAsnLeuArgGluThrHisSerLeuPhePheArg                               354045                                                                         SerLeuLysAspArgGlyPheValLeuThrPheLysThrAlaAspAsp                               505560                                                                         ProSerLeuSerLeuIleLysTyrGlyGluPheLeuTyrAspAsnLeu                               65707580                                                                       IleValPheSerProSerValGluAspPheGlyGlyAsnIleAsnVal                               859095                                                                         GluThrIleSerThrPheIleAspGlyGlyGlySerValLeuValAla                               100105110                                                                      AlaSerSerAspIleGlyAspProLeuArgGluLeuGlySerGluCys                               115120125                                                                      GlyIleGluPheAspGluGluLysThrAlaValIleAspHisHisAsn                               130135140                                                                      TyrAspValSerAspLeuGlyGlnHisThrLeuIleValAlaAspThr                               145150155160                                                                   GluAsnLeuLeuLysAlaProThrIleValGlyLysSerSerLeuAsn                               165170175                                                                      ProIleLeuPheArgGlyValGlyMetValAlaAspProAspAsnPro                               180185190                                                                      LeuValLeuAspIleLeuThrGlySerSerThrSerTyrSerPhePhe                               195200205                                                                      ProAspLysProIleThrGlnTyrProHisAlaValGlyLysAsnThr                               210215220                                                                      LeuLeuIleAlaGlyLeuGlnAlaArgAsnAsnAlaArgValIlePhe                               225230235240                                                                   SerGlySerLeuAspPhePheSerAspAlaPhePheAsnSerAlaVal                               245250255                                                                      GlnLysAlaThrProGlySerGlnArgTyrProGlnThrGlyAsnTyr                               260265270                                                                      GluLeuAlaValAlaLeuSerArgTrpValPheLysGluGluGlyVal                               275280285                                                                      LeuArgValGlyProValSerHisHisArgValGlyGluLysAlaPro                               290295300                                                                      ProAsnAlaTyrThrValThrAspLeuValGluTyrSerIleValIle                               305310315320                                                                   GluGlnLeuSerGlnGlyArgTrpValProPheAspGlyAspAspIle                               325330335                                                                      GlnLeuGluPheValArgIleAspProPheValArgThrPheLeuLys                               340345350                                                                      ArgLysGlyGlyLysTyrSerValGlnPheLysLeuProAspValTyr                               355360365                                                                      GlyValPheGlnPheLysValAspTyrAsnArgLeuGlyTyrThrHis                               370375380                                                                      LeuTyrSerSerThrGlnValSerValArgProLeuGlnAlaHisAla                               385390395400                                                                   ValArgAlaLeuHisProLeuGlyLeuProLeuLeuArgGlnArgLeu                               405410415                                                                      LeuHisAspGlyArgAlaLeuHisLeuGlnArgArgLeuLeuAlaHis                               420425430                                                                      GluGlyGluGlyGluVal                                                             435                                                                            __________________________________________________________________________ 

What we claim is:
 1. A method of expressing in a host cell a porcine apamin binding protein that is isolatable from porcine tissue, the method comprising transforming or transfecting the host cell with an oligonucleotide comprising a nucleic acid encoding porcine apamin binding protein, wherein the nucleic acid hybridizes under medium stringency conditions with a nucleic acid encoding the amino acid sequence of SEQ ID NO:2.
 2. The method of claim 1 in which the oligonucleotide comprises a nucleic acid having the sequence of SEQ ID NO:1. 